The invention relates to heat exchanger elements, in particular, for equipping heat exchangers for flue gas cleaning systems of power stations that are frequently equipped with a rotor which comprises a plurality of chambers for accommodating individual heat exchanger elements. The heat exchangers in a rotary implementation are frequently of the so-called Ljungström type. In the case of heat exchangers utilizing a stationary heat accumulator mass (stator), a design according to the so-called Rothemühle principle is frequently employed. Here too, the heat exchanger elements are inserted separately into chambers.
The heat exchanger elements have a honeycomb body consisting of a plastics material which is preferably matched to the geometry of the chambers. The honeycomb body comprises a plurality of mutually parallel flow channels which are separated from each other by channel walls and extend from one end face of the honeycomb body to the opposite end face.
Heat exchanger elements of the type mentioned hereinabove for employment in flue gas cleaning systems of power stations are known from the German patent specification DE 195 12 351 C1 for example. The heat exchanger elements disclosed therein are manufactured from reclaimed polytetrafluoroethylene alone or in a mixture with another plastics material and optionally they contain fillers.
The heat exchanger elements according to the invention are envisaged in particular for employment in so-called Ljungström heat exchangers and heat exchangers according to the Rothemühle principle. When employed in flue gas desulphurizing systems (REA), clean and raw gas flows are fed spatially separated in opposite directions through the heat exchanger/rotor which is equipped with the heat exchanger elements. In the region in which the raw or flue gas flows through the heat exchanger (rotor/stator), the heat exchanger elements are heated and the raw or flue gas is thereby cooled. In the region in which the clean gas flows through the heat exchanger (rotor/stator) in the reverse direction, the heat exchanger elements deliver energy to the clean gas whereby the temperature thereof rises whilst the heat exchanger elements then cool down again.
During the process of cooling the raw or flue gases, they can reach a temperature below the so-called dew point (TD) below which the water vapor contained in the raw or flue gas condenses and, together with fractions of SO3, HF and HCl, settles on the surfaces of the heat exchanger elements in the form of a highly corrosive mixture. The position within a heat exchanger from which the cooled raw or flue gases emerge wherein the temperature may possibly fall below the dew point TD is referred to as the cold end position. The cold end position can be in the lower region of the rotor (lower cold end position) or in the upper region of the rotor (upper cold end position) in dependence upon whether the flue gas is supplied from the upper end or the lower end of the rotor.
Consequently, apart from the temperature resistance demanded of the heat exchanger elements employed in these regions of the heat exchanger, a very high corrosion resistance is also required. Since the highly corrosive precipitate, typically mixed with ash residues, has to be regularly removed from the heat exchanger elements, easy handling and an efficient way of cleaning the heat exchanger elements are likewise of great economic importance. These requirements are met in satisfactory manner by the heat exchanger elements manufactured from plastics material.
Nevertheless, over long periods of operation, there has proved to be a problem under the given conditions that the heat exchanger/rotors which are typically manufactured from highly corrosion resistant steel remain in contact with the corrosive precipitates for a long period of time in the cold end positions that are equipped with the heat exchanger elements, and, due to the changing temperature conditions, they are inclined to corrode which requires that the heat exchanger parts and in particular the chamber walls be regularly replaced during the long lifetime of the heat exchangers. Hence, due alone to the stoppage of the heat exchangers entailed thereby, there are substantial economic costs and in addition to this, there are also the costs of the actual repair of the heat exchanger.
In the prior art, one has already tried to counter this problem by using an enamel coating on the heat exchanger parts. However, this has not proved to be sufficient in many cases.
Heat exchanger rotors have been proposed in WO 2013/127594 A1 wherein carbon and graphitic materials have been resorted to. This solution is comparatively expensive however.
The object of the invention is it to propose a heat exchanger element in which at least the tendency of the heat exchangers (rotors/stators) and in particular the chamber walls thereof to corrode is reduced and consequently the intervals between the individual repairs can be prolonged and possibly even the overall lifetime of the heat exchangers (rotors/stators) can also be prolonged so that they become substantially more economical to operate.